1. Field of the Invention
The present invention relates to a signal receiving method and a signal receiving apparatus suitable for application to a radio telephone system, for example.
2. Description of the Related Art
In a mobile communication such as a radio telephone system or the like, a multiple access in which a plurality of mobile stations (terminal apparatus) or subscribers are permitted to access a single base station is employed. In this case of a radio telephone, a number of mobile stations commonly utilize a single base station. Therefore, various communication systems have been proposed for avoiding interference between respective mobile stations. For example, a frequency division multiple access system (FDMA: Frequency Division Multiple Access), a time division multiple access system (TDMA: Time Division Multiple Access), a code division multiple access system (CDMA: Code Division Multiple Access) and so on are conventionally proposed as this kind of communication systems.
Of these systems, the CDMA system is a multiple access system in which a particular code is assigned to each of the mobile stations, a modulated wave of an identical wave (carrier) is spread in spectrum with the code and then transmitted to the identical base station, and a reception side takes code synchronism based on each code to identify a desired mobile station.
Specifically, the base station occupies the whole frequency band owing to the spread spectrum, and transmits signals to respective mobile stations using an identical frequency band at the same time. Each of the mobile stations inversely spreads a signal of a fixed spread band width transmitted from the base station to extract a corresponding signal. Further, the base station discriminates each of the mobile stations from one another by different spread codes.
In the CDMA system, communication can be achieved at every direct calling so long as a code is shared. Further, the system is excellent in maintaining secrecy of a telephone conversation. Therefore, the system is suitable for a radio transmission utilizing mobile stations such as a portable telephone apparatus and so on.
In the CDMA system, it is difficult to establish a precise communication relationship between mobile stations. Therefore, each communication between respective mobile stations cannot be dealt with completely separately, and hence another mobile station can become a source of interference upon communication with a mobile station. Further, data is spread within a particular frequency band in this system. Therefore, it is necessary to define a band width in advance over which the data is spread (i.e., a band width for use of transmission). Therefore, it is difficult to change the transmission band width.
The above matter will be described more concretely. FIGS. 1A and 1B show a model in which a transmission signal of a particular user is extracted by an inverse spread from transmission signals of eight mobile stations (users) which are spectrum spread with predetermined codes and multiplexed, for example. As shown in FIG. 1A, if a signal of a user U0 is to be extracted by the inverse spread from signals of eight users U0 to U7 which are multiplexed with codes, then as shown in FIG. 1B, the signal of the user U0 can actually be extracted. However, signals of other users U1 to U7 which are dealt with by the same base station also become an interference source, serving as a noise. This fact results in deterioration of an S/N characteristic. For this reason, in a radio transmission employing the CDMA system, the electric wave does not reach well due to the deterioration caused by the interference, which fact narrows a service area. Further, interference due to other users can be suppressed only by an amount of inverse spread gain which is obtained in a process of spectrum inverse spread. Therefore, a number of users (mobile stations) permitted to access is limited and a channel capacity becomes small.
Furthermore, in a communication system in which this kind of multiple access is carried out, it is important to have uniform sending powers of respective transmission signals present at a time so as to fall within a constant range, in order to suppress interference due to other users. However, in the conventional communication system in which the multiple access such as the CDMA or the like is carried out, a processing for controlling the sending power has been not always carried out satisfactorily.
Specifically, when a sending power of a signal from a certain terminal apparatus is adjusted to fall within a constant range, the base station side receives the signal transmitted from the terminal apparatus and detects its transmission state. Then, control data of the transmission output based on the result of the detection is transmitted to the terminal apparatus. Then, the terminal apparatus side determines the transmission state based on the transmitted control data and carries out processing for adjusting the transmission output to a corresponding state.
Now, FIG. 2 shows an example of a conventional arrangement for detecting a transmission state based on a received signal (this example is not an example of an arrangement peculiar to the CDMA system but a general arrangement for receiving a differentially modulated signal). For example, a received signal is supplied to an AGC circuit (automatic gain control circuit) 1 in which the signal is made into a signal having a gain within a constant range. An output of the AGC circuit 1 is supplied to a differential demodulating circuit 2 in which it is demodulated, and its demodulated output is supplied to a symbol deciding circuit 3. An output of the symbol deciding circuit 3 and the output of the demodulating circuit 2 are supplied to a subtracter 4 in which the difference between both the signals is detected. The detected difference becomes an estimated value of noise power. In this case, the output of the subtracter 4 is squared by a squaring circuit 5 to produce an absolute value. An output thereof is averaged by an averaging circuit 6 to calculate a mean value of the noise power.
However, the received signal should be adjusted to a constant level by the AGC circuit for detecting the noise power with precision. When the interference power is fluctuated due to interference or the like, it is difficult to adjust the level by the AGC circuit with precision, and hence it is difficult to estimate the noise power accurately.